(1) Field of the Invention
The present invention relates to the field of nuclear medicine. Particularly, the present invention relates to the field of transmission scanning to provide nonuniform attenuation correction within a gamma camera system.
(2) Background of the Invention
Non-uniform photon attenuation is an important factor that affects the quantitative accuracy of images collected using Single Photon Emission Computerized Tomography (SPECT) camera systems and can decrease the sensitivity of these systems for lesion detection. Non-uniform photon attenuation creates image degradation by interfering with and partially absorbing the radiation emitted from an organ containing a radio-pharmaceutical. Photon attenuation within SPECT systems tends to degrade images by also introducing image artifacts and other distortions that can result in false positive detection of lesions or the failure to detect lesions. The effects of photon attenuation are especially complex in cardiac studies as a result of nonuniform attenuation attributed to the thorax.
In transmission scanning, the source of radiation is directed toward the associated scintillation detector through the object of interest or patient. If the radiation field is significantly larger than the patient, the radiation source is allowed to directly radiate the detector, causing a high count rate in the scintillation detector. Those parts of the detector that become directly radiated are called unobstructed portions of the detector. It is not advantageous to allow large unobstructed detector areas because the resultant increase in count rate can lead to image degradation and in some cases the event detection electronics and processes can become overloaded (e.g., due to pulse pile-up) and temporarily terminate operation. These high count rates tend to reduce the imaging performance of the imaging system by loading down the signal detection and processing circuitry of the gamma camera.
Transmission computed tomography (TCT) can be used as a method for generating a nonuniform attenuation correction distribution. The transmission image data is gathered using a known source (e.g., line, sheet, or flood) of radiation. If performed separately from the SPECT emission study, the collection of the transmission data requires additional data acquisition time and the collection of the transmission and emission data is susceptible to misregistration effects due to patient (e.g., "object") movement between the data gathering sessions.
The transmission study may be performed simultaneously with the emission study. Among other advantages, simultaneous acquisition of both transmission and emission data reduces the effects of misregistration. In the case of a gamma camera with a single scintillation detector, it is known to use a sliding window or "band" associated with the field of view of the scintillation detector to move in conjunction with the line source to aid in allowing the gamma camera to differentiate between detected transmission photons and emission photons. For example, reference is made to an article entitled, "A Scanning Line Source for Simultaneous Emission and Transmission Measurements in SPECT," by Patrick Tan, et al., published in the Volume 34, No 10, of the Journal of Nuclear Medicine, in October 1993, which is incorporated by reference herein. This reference discloses use of a single scanning line source with a single moving detection band.
However, this solution offered by Tan et al. does not adequately account for "side scatter" or "cross-talk" in emission studies involving two scintillation detectors (e.g., within dual head detector systems). Cross-talk in emission studies involves transmission photons scattering off of an object (e.g., cause by Compton scatter) being studied and improperly detected by a scintillation detector as emission photons. In a dual detector gamma camera, transmission photons emitted from a line source that is associated with a given detector may be improperly detected (e.g., after scatter) as proper emission photons by the other detector. This is the case because the scattered photon loses energy as a function of the scatter angle and changes energy level. Cross-talk may also occur from emission photons that scatter off the object. In dual head gamma cameras, the effects of cross-talk are dealt with by performing a post processing operation on the detected data. This post processing operation is time consuming and not entirely accurate. Therefore, it would be advantageous to eliminate the need for such post processing step by eliminating the detection of the cross-scattered photons.
The use of a single sliding detection window associated with a single gamma camera detector does little to prevent cross-talk in a dual detector system. What is needed is a system that is operable within a dual detector camera system that effectively eliminates the improper detection of emission photons due to cross-talk. The present invention offers such a system and solution.
In addition, there is a dual detector gamma camera system that employs tracking zoom regions (e.g., window regions) that are designed to track the motion of an object of interest during ECT motion. Within this system, the zoomed regions of the detector change as the detector rotates through ECT motion about the object. Reference is made to U.S. Pat. No. 5,304,806, entitled, "Apparatus and Method for Automatic Tracking of a Zoomed Scan Area in a Medical Camera System," issued Apr. 19, 1994, and assigned to the assignee of the present invention, which discusses tracking zoom regions. The present invention provides for advantageous combination with the above system.
Accordingly, it is an object of the present invention to more effectively improve image quality within a nuclear medicine camera system. To this end, it is an object of the present invention to more accurately determine transmission and emission data within a gamma camera system. It is another object of the present invention to provide a mechanism and method for reducing the effects of cross-talk within a dual detector system during transmission and emission scanning sessions. It is another object of the present invention to provide such mechanisms and methods as described above that additionally allow effective use in conjunction with a tracking zoom region of a scintillation detector or tracking zoom regions of a pair of scintillation detectors. These and other objects of the invention not specifically recited above will become clear within discussions of the present invention herein.